Apparatus to improve stability of an MRAM over process and operational variations

ABSTRACT

An apparatus for improving stability of a magnetoresistive random access memory over process and operational variations includes a current reference circuit that provides a reference current control signal to variable analog control circuitry. The variable analog control circuitry is connected to receive control signals from the current reference. The variable analog control circuitry generates a word current reference signal in response to the reference current control signal and further generates a source current reference signal in response to the reference current control signal. At least one word current source is connected to receive the word current reference signal. At least one sense current source is connected to receive the source current reference signal.

FIELD OF THE INVENTION

The present invention relates to magnetoresistive random access memory (MRAM), and, more particularly, to an apparatus for improving stability of MRAM over process and operational variations.

BACKGROUND OF THE INVENTION

Typical MRAM structures have a nonmagnetic layer sandwiched between two ferromagnetic films. The two ferromagnetic films are also known as magnetic thin films. The MRAM employs the magneto resistive properties of this structure to store data. In each storage element, an MRAM employs two lines, commonly termed a word line and a sense string, in order to detect the magnetization direction of these magnetic thin films. Each string comprises a magnetic thin film that serves as a memory element, and the word line generally addresses multiple sense strings. Magnetic thin films that have a parallel moment have a low resistance and are typically assigned the ‘1’ state. Magnetic thin films having an anti-parallel moment have a high resistance and are typically assigned the ‘0’ state, but may also be assigned to the ‘1’ state.

During a read operation, a word current passes through the word line causing the magnetic layers in the sense string to rotate, thereby changing the resistance in the sense string. A sense current passes through the sense string. A sense line receives the signal from the sense string. A differential amplifier compares the signal from the sense line to a reference line to determine whether a one resistance or a zero resistance is stored in the MRAM. A differential amplifier notes the change in voltage across the sense line to determine resistive state of a storage element.

Magnetoresistive random access memory integrated circuits (MRAM IC's) have sensitive analog amplifiers that require current sources that operate within a small operation range around a selected operating point. Unfortunately, a current source operation range can vary as, for example, due to widely fluctuating manufacturing characteristics of MRAM IC's. This leads to the need for a stable reference over changes in voltage, temperature and fabrication process to control the operation point.

A useful stable reference must be stable over all the operating ranges that the device will experience. MRAM devices are typically subjected to environments that exhibit temperature, voltage, and process variations. If the analog circuitry is not stable over these conditions, it could lead to undesired or unstable operation levels at the final circuits such as the word sources and sense sources.

Some attempted approaches to provide a stable reference involve using a reference voltage that is routed throughout the entire chip. In such cases, there may be a separate reference voltage for read operations and another reference voltage for write operations.

In contrast to other approaches, the present invention employs a current reference that is stable over the operating range of the MRAM device to control operating levels in selected circuitry.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for improving stability of a magnetoresistive random access memory over process and operational variations. A current reference circuit provides a reference current control signal to variable analog control circuitry. The variable analog control circuitry is connected to receive control signals from the current reference. The variable analog control circuitry generates a word current reference signal in response to the reference current control signal and further generates a source current reference signal in response to the reference current control signal. At least one word current source is connected to receive the word current reference signal. At least one sense current source is connected to receive the source current reference signal.

In other aspects of the present invention, the method provides that the tunable circuit is a bias control circuit.

In other aspects of the present invention, the method provides that the operating parameter is a word current.

In other aspects of the present invention, the method provides that the operating parameter is a sense current.

In other aspects of the present invention, the method provides a method to adjust an operating parameter of a plurality of magnetic random access memories each having a tunable circuit comprising: measuring the operating parameter of at least one of the magnetic random access memories to obtain a measured operating parameter result; and tuning at least one of the plurality of the tunable circuits based on the measured operating parameter result.

In other aspects of the present invention, the method provides that the tunable circuit is a bias control circuit.

In other aspects of the present invention, the method provides that the plurality of magnetic random access memories are constructed on a single wafer.

In other aspects of the present invention, the method provides that the operating parameter is a word current.

In other aspects of the present invention, the method provides that the operating parameter is a sense current.

In other aspects of the present invention, a bias tuning circuit for a MRAM includes a bias generator having a bias output; a plurality of switches having a word reference input and a mirror transistor output; a plurality of mirror transistors connected to one of the mirror transistor outputs; a transistor connected in a mirror configuration with the plurality of mirror transistors and having a tuned reference output; a selector to select one of the mirror transistor to activate the transistor to set the voltage to the plurality of mirror transistors.

In other aspects of the present invention, each one of the plurality of mirror transistors has a different gain.

In other aspects of the present invention, the bias tuning circuit further comprises a pad and an indicator transistor in a mirror configuration with the transistor connected to the pad to provide an indicator.

In other aspects of the present invention, the pad is an external pad.

In other aspects of the present invention, the indicator transistor has a gain that is a multiple of the transistor.

In other aspects of the present invention, the plurality of mirror transistors are n-channel transistors.

In other aspects of the present invention, the plurality of switches are transistors.

In other aspects of the present invention, the transistor is an N-channel transistor.

In other aspects of the present invention, the bias generator is a temperature and voltage compensated bias generator.

In other aspects of the present invention, the selector selects one of the plurality of mirror transistors to compensate for a tested parameter.

In other aspects of the present invention, the tested parameter is a manufacturing variance.

The present invention also provides, in the most preferred aspects, a current controlled word current source comprising: a current source having a stable reference current output; and a word current source having a word current reference input connected to the stable reference current output, with the word current source having a word current output.

In other aspects of the present invention, the control circuit comprises a voltage regulator that regulates a voltage level at the gate, so as to limit an amount of current flowing through the n-channel transistor and the magnetoresistive random access memory circuit, and the voltage regulator comprising a feed back amplifier.

In other aspects of the present invention, the voltage regulator is current controlled.

In other aspects of the present invention, the n-channel transistor comprises a complementary metal oxide semiconductor n-channel field effect transistor.

In other aspects of the present invention, the control circuit comprises a current regulator that regulates current flowing through the complementary metal oxide semiconductor (CMOS) n-channel field effect transistor when the CMOS n-channel field effect transistor is turned on and off.

In other aspects of the present invention, the current regulator comprises a feed-back amplifier.

In other aspects of the present invention, the control circuit comprises a stabilization amplifier.

In other aspects of the present invention, the stabilization amplifier further comprises a logic control having a read/write input, an on/off input, a write reference gate control signal and a read reference gate control signal; a read reference switch having a read reference input and a reference output, with the read reference switch having a read reference control connected to the read reference gate control signal; a write reference switch having a write reference input and a write reference output connected to the reference output, with the write reference switch having a write reference control connected to the write reference gate control signal; and a feedback amplifier connected to the reference output and having a mirror current output and a mirror feedback voltage input.

In other aspects of the present invention, the control input is a regulated mirror gate signal.

In other aspects of the present invention, a word current source for a magnetoresistive random access memory circuit comprises: a control circuit having a regulated mirror gate signal; a complementary metal oxide semiconductor (CMOS) n-channel transistor including a gate, a source and a drain, where the source is coupled to a supply ground, and the drain is coupled to the magnetoresistive random access memory circuit; and a positive supply voltage coupled to the magnetoresistive random access memory circuit so as to allow current to flow through the magnetoresistive random access memory circuit when an activation signal is applied to the gate by the control circuit, wherein the control circuit comprises means for regulating the voltage level at the gate, so as to limit the amount of current flowing through the CMOS n-channel transistor and the magnetoresistive random access memory circuit.

In other aspects of the present invention, the control circuit comprises means for regulating a current flowing through the CMOS n-channel transistor when the CMOS n-channel transistor is turned on and off.

In other aspects of the present invention, the control circuit further comprises a stabilization amplifier.

In other aspects of the present invention, a magnetoresistive random access memory circuit comprises a control circuit having a control input; an n-channel semiconductor device including a first terminal, a second terminal and a third terminal, where the first terminal is coupled to a supply ground, and the second terminal is coupled to the magnetoresistive random access memory circuit; and a positive supply voltage coupled to the magnetoresistive random access memory circuit so as to allow current to flow through the magnetoresistive random access memory circuit when an activation signal is applied to the third terminal by the control circuit.

In other aspects of the present invention, the control circuit comprises means for regulating the voltage level at the third terminal, so as to limit the amount of current flowing through the n-channel semiconductor device and the magnetoresistive random access memory circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of an MRAM segment utilizing preferred methods according to the preferred teachings of the present invention, with portions of the MRAM structure removed to show details of the noise stabilization and reduction apparatus of the present invention.

FIG. 2A shows an end view of a sense string and word line, with portions of the MRAM structure removed to show details of the structure of the sense string and word line.

FIG. 2B shows a side view of a sense string and word line, with portions of the MRAM structure removed to show details of the structure of the sense string and word line.

FIG. 3 shows a simplified circuit seen by the differential amplifier utilizing preferred methods according to the preferred teachings of the present invention with a sense string and a word line active.

FIG. 4A shows a schematic circuit diagram of a magnetoresistive random access memory integrated circuit (MRAM IC) including a current reference for stabilizing word current sources and sense current sources as contemplated by the present invention.

FIG. 4B shows a sense current source circuit according to the preferred teachings of the present invention.

FIG. 5 shows a schematic circuit diagram of a word current source for use in an MRAM using an n-channel device constructed in accordance with the preferred teachings of the present invention.

FIG. 6 shows a circuit schematic diagram of a feedback based current driver according to the preferred teachings of the present invention.

FIG. 7 shows a circuit schematic diagram of a current driver according to the preferred teachings of the present invention.

FIG. 8 shows a circuit schematic of a bias/reference circuitry, the bias setting circuitry and indicator circuitry and external pad according to the preferred teachings of the present invention.

FIG. 9 shows a method to test an MRAM and set MRAM operating points according to the preferred teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A top view of an MRAM segment having a current controlled word and sense source, according to the preferred teachings of the present invention, is shown in FIG. 1 and is generally designated 10. Portions of the MRAM structure shown in FIG. 1 have been removed to show details more clearly. Those skilled in the art will be aware that MRAM chips contain other structures and layers, such as a transistor layer that may be formed from polysilicon and a metal connect layer. These elements have been removed for the sake of clarity.

The MRAM segment includes a plurality of sense strings 20, 22, 24, 26. Each sense string 20, 22, 24, 26 includes one or more sub bits 30, 32, 34, 36, 38, 40, 42, 44 connected by strap layer segments 50, 52, 54, 56, 58, 60, 62, 64, 66. In the preferred embodiment of the present invention, the strap layer segments 50, 52, 54, 56, 58, 60, 62, 64, 66 connect the sub bits 30, 32, 34, 36, 38, 40, 42, 44 in series. In further aspects of the preferred embodiment, the structure of the sense strings 20, 22, 24, 26 have a serpentine conformation. In this conformation, groups of two sub bits 30, 32, 34, 36, 38, 40, 42, 44 form linear components. The strap layer segments 50, 52, 54, 56, 58, 60, 62, 64, 66 provide connection elements to join the sub bits 30, 32, 34, 36, 38, 40, 42, 44 into these linear components. Four of these linear components are located parallel to one another. The strap layer segments 50, 52, 54, 56, 58, 60, 62, 64, 66 also provide connection elements to join the linear components at alternating ends in order to connect the sub bits 30, 32, 34, 36, 38, 40, 42, 44 in series. In the preferred embodiment, the sense strings 20, 22, 24, 26 include eight sub bits 30, 32, 34, 36, 38, 40, 42, 44 connected in series. In an alternative embodiment, the sense strings 20, 22, 24, 26 may make up a single sub bit. Different numbers of sub bits and as well as different arrangements of the sub bits may be employed without departing from the spirit and scope of the invention.

The sub bits 30, 32, 34, 36, 38, 40, 42, 44 include the data storage element of the MRAM segment 10. These sub bits 30, 32, 34, 36, 38, 40, 42, 44 may also be termed “memory spots” or “memory elements”. In the preferred embodiment, the sub bits or memory spots 30, 32, 34, 36, 38, 40, 42, 44 are grouped in fours, where the upper four memory spots 38, 40, 42, 44 make up an upper bit 70 and the lower four memory spots 30, 32, 34, 36 make up a lower bit 72.

The MRAM segment 10 employs a word line 80, 82, 84, 86 to address a selected bit 70, 72. In the preferred embodiment, the MRAM segment 10 uses two word lines 80, 82 to address the sense strings 20, 22, 24, 26, with an upper word line 80 addressing the memory spots 38, 40, 42, 44 of the upper bit 70 and a lower word line 82 addressing the memory spots 30, 32, 34, 36 of the lower bit 72. The upper word line 80 intersects each of the upper sub bits 38, 40, 42, 44 so that a sense current passing through the upper sub bits 38, 40, 42, 44 is orthogonal to a word current passing through the upper word line 80. Likewise, the lower word line 82 intersects each of the lower sub bits 30, 32, 34, 36 so that a sense current passing through the lower sub bits 30, 32, 34, 36 is orthogonal to a word current passing through the lower word line 82. Serial connection of the memory spots 30, 32, 34, 36, 38, 40, 42, 44 combined with activation of the word line 80, 82, 84, 86 corresponding to a selected bit 70, 72 allows each sub bit 30, 32, 34, 36, 38, 40, 42, 44 of the selected bit 70, 72 to contribute proportionally to the signal size.

As those skilled in the art will understand, other conformations of the sense strings 20, 22, 24, 26 may be employed without departing from the spirit or scope of the invention. In the four memory spot bit described above, each memory spot, or sub bit 30, 32, 34, 36, 38, 40, 42, 44 is designed to have length to width ratio providing for consistent switching characteristics. In one aspect of the invention, the number of memory spots 30, 32, 34, 36, 38, 40, 42, 44 per bit 70, 72 is designed to provide a selected signal size. In another aspect of the present invention, the number of memory spots 30, 32, 34, 36, 38, 40, 42, 44 per bit 70, 72 is designed to provide redundancy in the event of a defective bit. The defective bit may be the result of a manufacturing defect or operational failure. The MRAM may be advantageously designed to have functional bits with only three of four memory spots operational. In another embodiment, the MRAM may be advantageously designed to have functional bits with only two of three memory spots operational.

In other aspects of the present invention, the multiple memory spots 30, 32, 34, 36, 38, 40, 42, 44 of the bit 70, 72 may be addressed by a single word line 80, 82, 84, 86 to conserve power and allow a higher density of bits 70, 72; or alternatively, multiple word lines 80, 82, 84, 86 may be used to address the multiple memory spots 30, 32, 34, 36, 38, 40, 42, 44 of the bit 70, 72 when more memory spots 30, 32, 34, 36, 38, 40, 42, 44 are desired to obtain a stronger signal to noise ratio or a higher level of redundancy.

In a typical MRAM structure, an array 90 of sense strings includes multiple sense strings 20, 22 positioned adjacent to one another in a linear arrangement. These sense strings 20, 22 have the same general shape, so that the word line 80, 82 may address the sub bits 30, 32, 34, 36, 38, 40, 42, 44 of each sense string in the array 90. In one preferred embodiment, the array 90 includes thirty-three sense strings 20, 22 that may each be addressed by the upper word line 80 and the lower word line 82. As those skilled in the art will understand, the word line 80, 82 may address more or fewer sense strings 20, 22 without departing from the spirit or scope of the present invention. The sub bits 30, 32, 34, 36, 38, 40, 42, 44 of each sense string 20, 22 must be positioned so that a sense current passing through the sub bits 30, 32, 34, 36, 38, 40, 42, 44 is orthogonal to a word current passing through the word line 80, 82.

Each sense string 20, 24 has an input end 100, 102, 104, 106 connected to a voltage source 108 through a switch 110, 112, 114, 116. A transistor may serve as the switch 110, 112, 114, 116. A signal 118 triggers the switch 110, 112, 114, 116 of a selected sense string 20, 22, 24, 26 to allow a sense current to pass through the respective sense string 20, 22, 24, 26. Each sense string 20, 22, 24, 26 also has an output end 120, 122, 124, 126 connected to a sense line 128, 130. In the preferred form, the MRAM segment 10 includes two sense lines, an upper sense line 128 and a lower sense line 130, respectively. The MRAM segment 10 further includes two arrays 90, 92 of sense strings 20, 22, 24, 26, an upper array 90 positioned above the two sense lines 128, 130 and a lower array 92 positioned below the two sense lines 128, 130.

The MRAM segment 10 of the preferred form of the present invention provides for noise stabilization and reduction through the coupling of the respective output ends 120, 122, 124, 126 of the sense strings of the upper array 90 and the lower array 92. In one example embodiment, shown in FIG. 1, the output end 120, 122 of each of the sense strings 20, 22 of the upper array 90 is connected alternately to the upper sense line 128 and the lower sense line 130. Thus, in this example embodiment, sense string 20 is connected to the lower sense line 130, and sense string 22 is connected to the upper sense line 128. Likewise, the output end 120, 122, 124, 126 of each of the sense strings 24, 26 in the lower array 92 is connected alternately to the upper sense line 128 and the lower sense line 130. In this example embodiment, sense string 24 is connected to the upper sense line 128 and sense string 26 is connected to the lower sense line 130. This pattern of coupling the output ends 120, 122, 124, 126 of the sense strings 20, 22, 24, 26 continues for each of the sense strings 20, 22, 24, 26 in the arrays 90, 92. Those skilled in the art will understand that other patterns of coupling the output ends 120, 122, 124, 126 of the sense strings 20, 22, 24, 26 may be employed without departing from the spirit or scope of the present invention.

The upper sense line 128 and the lower sense line 130 provide the signal from the sense strings 20, 22, 24, 26 to a differential amplifier 132. The differential amplifier 132 detects the voltage difference in the signal provided by the upper sense line 128 and the lower sense line 130. Determination of the state of a selected bit makes use of the output of the differential amplifier 132.

FIGS. 2A and 2B show an end view and a side view, respectively, of a sense string 20, 22, 24, 26 and word line 80, 82, with portions of the MRAM structure removed to show details of the structure of sense string 20, 22, 24, 26 and word line 80, 82. The MRAM segment 10 has a strap layer 200 and a bit layer 202 embedded within a dielectric layer 204. The dielectric layer 204 also serves as an insulating layer 204. The sub bits 30, 32, 34, 36, 38, 40, 42, 44 are formed from sections of the bit layer 202 embedded within the dielectric layer 204. As shown in FIGS. 2A and 2B, the strap layer 200 overlies the bit layer 202. The strap layer 200 provides connection elements between the sub bits 30, 32, 34, 36, 38, 40, 42, 44. Overlap between the strap layer 200 and the sub bits 30, 32, 34, 36, 38, 40, 42, 44 provide contact between the strap layer 200 and the sub bits 30, 32, 34, 36, 38, 40, 42, 44. The word lines 206 are also embedded within the dielectric layer 204, and in the preferred form, the sense strings 20, 22, 24, 26 overlie the word lines 206. The conformation of the word lines 206 and the sense strings 20, 22, 24, 26 become a source of capacitive coupling. Furthermore, in order to present a substantially uniform field to the sub bits 30, 32, 34, 36, 38, 40, 42, 44, the length of the sub bits may be limited to the width of the word lines 206.

The present invention provides for a greater signal differential by employing multiple sub bits 30, 32, 34, 36, 38, 40, 42, 44 for each bit 70, 72. The memory spots for each bit are set to have the same magnetization state. Thus, in a high resistance state, or “0” state, the difference in resistance from a low resistance state, or “1” state, will be proportional to the number of memory spots 30, 32, 34, 36, 38, 40, 42, 44 in a bit 70, 72. In the preferred embodiment, sub bits 30, 32, 34, 36 and sub bits 38, 40, 42, 44 each make up one bit 70, 72, respectively. By connecting these memory spots 30, 32, 34, 36, 38, 40, 42, 44 in series, the example embodiment shown provides a signal having a voltage drop four times the magnitude that would be provided from a single memory spot. More or fewer memory spots or sub bits 30, 32, 34, 36, 38, 40, 42, 44 may be employed for each bit 70, 72 to provide a signal having a desired magnitude.

The present invention also provides for a greater memory capacity by employing multiple groups of sub bits 30, 32, 34, 36, 38, 40, 42, 44 on each sense string 20, 22, 24, 26. Each group of sub bits 30, 32, 34, 36, 38, 40, 42, 44 on the sense string 20, 22, 24, 26 make up a separate bit 70, 72 and has a separate word line 80, 82, 84, 86 so that each group of sub bits 30, 32, 34, 36, 38, 40, 42, 44 may be addressed separately. In the preferred embodiment of the present invention, the upper word line 80 addresses upper sub bits 38, 40, 42, 44 and the lower word line 82 addresses lower sub bits 30, 32, 34, 36. A word current through either word line 80, 82 addresses the respective sub bits while not appreciably changing the resistance of the other sub bits. More or fewer groups of sub bits may be employed without departing from the scope of the present invention.

The MRAM queries the state of a bit using a sense current and a word current. By way of example, and not limitation, determination of the lower bit 72 begins by sending a signal that triggers the switches 110, 114 for the first sense string 20 and the reference sense string 24. This allows a sense current from voltage source 108 to flow through each respective sense string 20, 24. Concurrently, the MRAM applies a word current through the lower word line 82 of the upper array 90. All other sense strings 22, 26 and word lines 80, 84, 86 remain inactive. The magnetic field from the word current change the resistance of the sub bits 30, 32, 34, 36 to the sense current. By way of example, the current through the sense strings 20, 24 can be on the order of 3-5 milliamps and the current passing through the word line 82 can be on the order of 40-50 milliamps. These values are representative and may vary.

In the foregoing example, the lower sense line 130 receives the sense current from the sense string 20 and serves as a reference sense line. A second sense string, reference sense string 24, acts as a reference for sense string 20 and provides a reference signal unaffected by a word current. An upper sense line 128 receives the sense current through sense string 24. In a similar fashion, when the MRAM segment 10 addresses a bit on sense string 24, sense string 20 may serve as a reference. The differential amplifier 132 samples the signals from the upper sense line 128 and the lower sense line 130. The differential amp 132 includes circuitry to employ an auto zero technique that locks in the difference of the signals from the upper sense line 128 and the lower sense line 130 as a base value. The current on the word line 82 is then reversed, causing the resistance of the memory spots 30, 32, 34, 36 to change because of the change of the magnetic field generated by the word line 82. The differential amplifier 132 then samples the signals from the upper sense line 128 and the lower sense line 130 again and provides the results to a comparator. The differential amplifier 132 further includes a comparator to determine the state of the lower bit 72.

In the foregoing example, the differential amplifier 132 receives a signal from the sense string 20 on the lower sense line 130 and a signal from reference sense string 24 on upper sense line 128. However, in addition to the signal from the sense current passing through the sense string 20, the current from the word line 82 has a capacitive interconnect with the sense string 20 and each of the sense strings 22 in the same array as the sense string 20. The capacitive interconnects generate a significant amount of noise in comparison to a bit component of the signal from the sense string, comprising up to fifty percent of the bit component. Furthermore, the noise generated by the capacitive interconnects between the word line 82 and the sense strings 20, 22, 24, 26 vary between each sensing event. Also, the noise is proportional to the number of sense strings 20, 22 in the array 90. Thus, as the array size increases, the amount of noise due to capacitive interconnects increases proportionally. These noise levels are a major impediment to development of fast and reliable MRAM applications. With increasing MRAM array sizes, these hindrances are exacerbated.

The MRAM segment 10 according to the preferred teachings of the present invention stabilizes and reduces noise generated by these capacitive interconnects. By coupling a first portion of each array 90 of sense strings to the upper sense line 128 and a second portion of each array 90 of sense strings 20, 22, 24, 26 to the lower sense line 130, the MRAM segment 10 reduces the amount of noise seen by each sense line 128, 130 proportional to the portion of sense strings 20, 22, 24, 26 coupled to the other sense line 128, 130. In the preferred embodiment, alternating sense strings 20, 22, 24, 26 in an array 90, 92 are coupled the upper sense line 128 and the lower sense line 130, respectively, reducing the amount of noise from capacitive coupling by approximately fifty percent. MRAM segment 10 according to the preferred teachings of the present invention also stabilizes the effect of noise through cross coupling of the sense strings 20, 22, 24, 26. The cross coupling of the sense strings 20, 22, 24, 26 balances the noise generated in the sense strings 20, 22, 24, 26 by activation of the word line 80, 82 between the upper sense line 128 and the lower sense line 130.

FIG. 3 shows a simplified circuit seen by differential amplifier 132 with the sense string 20 and the word line 82 active. At one input, the differential amplifier 132 receives the sense signal 210 from a sense string 20 having an active word line 82 with a word current 212. The other input receives a reference signal 214 from the reference sense string 24. Both the sense signal 210 and the reference signal 214 include a sense current 216 and a noise current injected by the capacitive coupling. The difference seen by the differential amplifier 132 is now largely due to the different voltage drop across the sense string 20 with the active word line 82 because of the different resistance to the sense current 216. A second signal can be obtained by reversing the word current 212.

Referring now to FIG. 4A, there shown is a schematic circuit diagram of a magnetoresistive random access memory integrated circuit (MRAM IC) chip including a current reference for stabilizing word current sources and sense current sources as contemplated by the present invention. The MRAM IC chip 10 includes standard core circuitry 720, variable analog control circuitry 714, current reference 722, a word current source 724, and a sense current source 729. The current reference 722 is connected by electrical connections 730 for routing control signals from the current reference 722 to the variable analog control circuitry 714. The variable analog control circuitry 714 uses the control signals from the current reference 722 to maintain constant operation of the word current sources 724, sense current sources 729 and other selected circuitry. A reference output 733 is generated by the variable analog control circuitry 714. Variable analog control circuitry 714 is connected to the word source 724 by the word reference current 734. The sense source 729 is connected to the core circuitry by Is sense current signal 725. The word source 724 is connected to the core circuitry 720 by Iw word current signal 727.

Since these circuits may vary in a non-uniform way over operating conditions, adjustments may be needed in the output of the stable current reference or the analog control circuitry.

The current reference 722 may advantageously be constructed from several types. For example, the current reference 722 may advantageously include a bandgap voltage reference that feeds into and controls a stable current reference. Alternatively, the current reference 722 may comprise less precise devices or circuits that may include any of a variety of compensated, balanced sources. In one example embodiment of the present invention, the current reference itself, or the intermediate circuitry it feeds, may advantageously be adjustable so that the output signal of the reference matches the characteristics of the active, final circuitry. By producing such an adjustable reference signal, the MRAM circuitry would be constant over many operating conditions that the device will experience.

Refer now to FIG. 4B which shows the sense driver 712 electrical schematic diagram according to the preferred teachings of the present invention. The sense driver 712 has a P-channel transistor 720 that shares gate control voltage 732 with sense current drivers P-channel transistors 722 and 724. The input to transistor 720 is a read reference current 726. The transistor 720 is supplied by VDD. The sense current driver P-channel transistors 722 and 724 provide current on sense current lines 728 and 730, respectively, to the sense lines on the MRAM 10. The sense driver 712 is an example embodiment of the sense sources 729. The sense current lines 728 and 730 are example embodiments of Is sense current signal 725.

The reference signal 726 is a “long distance” signal that provides a current. The reference signal 726 is converted “locally” to a voltage that is stable and consistent, because the supply voltage VDD in the “local” area is the same, as well as the VSS in the “local” area. The “local” area is considered near the drive transistors 722 and 724. One example embodiment of the reference signal 733 is the reference current 726.

These references need to be transferred to the circuits that use them to set operation levels. There are three ways of doing this. One way is to use a voltage at a level between power and ground. The other way is to use a small current. A third way is a combination of voltages and currents.

If a voltage is routed its level must be closely controlled at all times with very small variations. This is difficult to do because any intersection with adjoining or crossing conductors could lead to variations. These variations would be caused by capacitive coupling of noise from the active signal lines to the reference line. This could lead to undesired or unstable operation levels at the final circuits such as the word and sense sources.

If a current is used to transfer the operation level to the word sources 724 and sense sources 729, it is affected much less than a voltage. This is because voltage is the mechanism of transferring or coupling noise to the reference lines. Only the C dv/dt=i (couple) is transferred to the reference, and the C is dependent upon the active line and all of the associated transfer circuitry. This is small in relation to the reference current and hence doesn't have much effect unless the reference current is set to low. The circuitry associated with transferring and receiving the current has an extended rc time constant, so any small variations tend to be minimized.

The sense current 216 through switch 110 and switch 114 can be developed with two identical drive transistors.

Referring now also to FIG. 5, there shown is a schematic circuit diagram of an example embodiment of word driver 724 and part of the core circuitry 720. A MRAM with an optimized MRAM current source system 435 using a word current source 463 is constructed using a regulated n-channel transistor 843 as contemplated by the present invention. One example embodiment of the word source 724 is the word current source 463. An alternate embodiment of the word source 724 is shown as the circuit of FIG. 7. The optimized current source system 435 includes a positive voltage supply VDD, a supply ground GND, n-channel controlling circuitry 450, MRAM circuitry 460 supplied by the regulated word current source 463 and the n-channel transistor 843. The positive voltage supply VDD is connected to a current input 461 of the MRAM circuitry 460. The n-channel transistor 843 includes a gate Gn, a drain Dn and a source Sn. The gate Gn is connected to the output 452 of the n-channel controlling circuitry 450, and the drain Dn is connected to a current output 462 of the MRAM circuitry 460. The source Sn is connected to the supply ground GND. When an activation signal is applied to gate Gn, current Iwrd flows through the MRAM circuitry 460 from VDD and through the n-channel transistor 843 into the supply ground GND. In this case, the n-channel controlling circuitry 450 regulates the voltage level of n-channel control and limits the amount of current fed through it and the other components. The n-channel controlling circuitry 450 also regulates the current when the n-channel transistor 843 and hence the regulating current source 463 itself is turned on and off. The stabilization amplifier 500 shown in FIG. 7 may be used to accomplish this function.

Provisions are suitably provided to use word current sources constructed using regulated n-channel transistors. N-channel transistors conduct more current per unit size and can be more precisely controlled. Thus, the drive transistor size, such as transistor 843, can be reduced by approximately ½ as compared to p-channel transistor devices, while maintaining tight control of the current word source 463. Word current sources 463 thus constructed in accordance with the invention are proportionally reduced in size, resulting in a substantial reduction in size for an MRAM chip 10 employing the word current sources 463 as contemplated by the present invention.

For the purpose of explaining the invention, it will be described herein with reference to example embodiments. It will be understood that the example embodiments are by way of illustrating the various aspects of the invention and that the invention is not limited by the examples. As contemplated by the present invention, the concept of using a regulated n-channel word current source design, as opposed to the old regulated p-channel current source based designs, is based on the word current source 463 being a circuit used to control the amount of current allowed to flow between positive voltage supply VDD and supply ground GND. While the word current source 463 is required to be stable independent of variables such as supply voltage, process variation and temperature, it doesn't matter where it is in the connection between voltage supply VDD and supply ground GND as long as the function is performed. Connecting the word current source 463 to supply ground GND is, therefore, a result of redefining the function of the word current source 463.

One feature of a word current source constructed in accordance with the present invention using a regulated n-channel based source transistor 843 is that the associated n-channel controlling circuitry 450 can also advantageously be smaller and consume less area on an MRAM 10 as compared to previous designs. This is true because the current source controlling circuitry 450 may be sized in proportion to the device being driven. Since an n-channel device is about half the size of a p-channel device, the n-channel controlling circuitry 450 can also be about half the size of a prior art p-channel current source control.

According to the preferred teachings of the present invention, smaller n-channel devices have less capacitance and can be turned on in less time, with greater control. This in turn leads to lower noise during operation, thereby increasing the reliability of an MRAM 10 constructed in accordance with the present invention.

Refer now also to FIGS. 6 and 7 which show one example embodiment of a word driver 724 according to the preferred teachings of the present invention. A read reference current 818 ranges from 50-150 microamps, and preferably 100 microamps, and a write reference current 820 ranging from 100-300 microamps, preferably 200 microamps, is provided to a stabilization and control block 886. Depending on the reference current selected, either the I ref write reference current 820 or the I ref read reference current 818 will be driven on the I ref signal line 851. An on/off signal 810 and 804, implemented as a double enable, and a read/write signal 802 provide control signals to the stabilization and control block 886.

Refer now to drawing 7 that shows the word drive circuitry with its associated controls and features. The read/write signal 802 is buffered and provided to NAND 809. The inverted version of read/write signal 802 is signal 812, and signal 812 is provided to NAND 811. A WLEN signal 810 and WLDEN signal 804 are provided to NAND 805. The WLDEN signal 804 indicates what direction, either a read direction or write direction, a word line current Iwrd 840 is driven. The output of NAND 805 is inverted and provided to NAND 809 as On1 signal 808. The On1 signal 808 is provided to a NAND 811 also. NAND 809 provides the enable signal 814 to the gate of P-channel transistor 823. Transistor 823 enables word reference write current 820 input and provides the VMI signal 851 when enabled. Iwrd 840 is an example embodiment of the Iw word current signal 727.

The On1 signal 808 is inverted to generate the OFF2 signal 845. NAND 811 controls the gate of P-channel transistor 821. Transistor 821 enables word reference read current 818 input and provides the VMI signal 851 when enabled.

Capacitor 846 is connected to the VMI signal as well as the MN off2 transistor 847. The gate of MN off2 transistor 847 is controlled by the Off2 signal 845. The Off2 signal 845 also controls the gate of the MPREF transistor 844.

The VMI signal is connected to control the gate of MNREFB 852. MNREFB 852 is connected to the MPREF transistor 844 and the MNMIR3 transistor 842. The MNMIR3 transistor 842 is connected in a mirror configuration to MNSOURCE transistor 843. The MNMIR1 transistor 848 is connected to the VMI signal and its gate is controlled by the VMH signal 850. The MNMIR2 transistor 849 is connected to the MNMIR2 transistor 848 and is gate controlled by the VMH signal 850 as well.

The MPEQ transistor 832 is gate controlled by the NEQUAL′ signal 720. The MPEQ transistor 832 drives the word line array through the word line current Iwrd 840 connection. The MNSOURCE transistor 843 also drives the word line current Iwrd 840. The NEQUAL′ signal 720 is generated by a double inverted drive combination from the WLEN signal 710. This is configured for the P-channel MPEQ transistor 832. The control for the MPSW switch transistor 830 takes into account the enable circuitry, and WLEN signal 710 and WLON signal 827 as well as the disable function provided by TestW 829 and its circuitry. The output of nor 826 is inverted and provided to the gate of the MPSW switch transistor 830. Nor 826 nors the TESTW signal 829 with the output of the NAND 824 of the WLEN signal 710 and the word line on control WLON signal 827.

The control of p-channel transistor MPSW 830 includes the TestW signal 829 and WLON signal 827 options. They allow for on and off control of p-channel transistor MPSW 830 independent of the control of the n-channel transistor MNSOURCE 843 and its controlling circuitry. The controlling circuitry of the n-channel transistor MNSOURCE 843 is shown in FIG. 7.

The independent WLON control signal 827 to NAND 824 allows the p-channel transistor 830 to be turned on or off at a different time than n-channel transistor 843. With this feature, the p-channel transistor MPSW 830 could be turned on prior to the n-channel transistor MNSOURCE 843. When this is done, current surges from switching can be reduced or regulated because switching is then controlled by the n-channel transistor MNSOURCE 843.

The TestW signal 829 that controls the nor 826 is the feature that allows an independent disable of p-channel transistor 830. This is independent of the normal function of the MRAM 10. During normal operation, the TestW signal 829 is held low and is disabled. When the TestW signal 829 is forced high to VDD, the function of p-channel transistor MPSW 830 can be supplied by an external connection or pad 861 connected to the word line current Iwrd 840. By observing the current or voltage on this external connection, the actual word line current Iwrd 840 can be measured and observed for magnitude and overshoot conditions.

The stabilization and control block 886 provides a central current signal 888 to the mirror input n-channel transistor 842. The stabilization and control block 886 accepts a feed back voltage 850. The stabilization and control block 886 dampens the effect of the change in the feed back voltage 850. The gates of the mirror output n-channel MNSOURCE transistor 843 and the mirror input n-channel MNMIR3 transistor 842 are connected. The mirror output n-channel transistor MNSOURCE 843 supplies the word line current I wrd 840.

The R/W signal 802 and on/off signals 810 and 804 are provided to a logic control block 504 that provides a gate control signal 814 to P-channel write transistor 823 and a gate control signal 508 to P-channel read transistor 821. The logic control block 504 implements the following truth table in a well-known manner using boolean logic: R/W On/Off PW PR On On Off On Off On On Off On Off Off Off Off Off Off Off where On is VDD and Off is VSS when applied to the gates of transistors 823 and 821.

One of the inputs to the word driver stabilization amplifier is the reference current I ref 851 that flows through transistor 848 and transistor 849 creating a voltage that controls transistor MNREFFB 852. The current through transistor 852 is the input to the MNMIR3 mirror transistor 842 that gets multiplied through MNSOURCE transistor 843 to become the word line current Iwrd 840. The current through MNSOURCE transistor 843 is dampened from overshoot by the interaction of the voltage on signal line 850 through an increasing voltage on signal line 850 that increases the conductance of MNMIR2 transistor 849 and MNMIR1 transistor 848 that reduces the voltage level of signal 851, that in turn reduces the conductance of MNREFFB transistor 852. This decreases the mirror reference current Im 888 into the mirror input MNMIR3 transistor 842 that reduces the overshoot in the word line current Iwrd 840 that in turn reduces the word current I wrd in MNSOURCE transistor 843.

The capacitor 846 reduces the rate of change of the voltage signal 851 and stabilizes the dampening process in the feedback loop between MNMIR7 transistor 848, MNMIR2 transistor 849, MNREFFB transistor 852, MNMIR3 transistor 842 and MNSOURCE transistor 843.

Functionally, any one or none of the reference signals, write reference current 820 or read reference current 818 will drive the current input signal 851 to a stabilization amplifier 500 implemented as a current amplifier. For example, with an on R/W signal 802 in the on state and an on/off signal 810 in the on state, the current or the current input signal originates from the P-channel transistor 821, providing a read reference current to the stabilization amplifier 500.

The word line current Iwrd 840 of the n-channel drive supply transistor 843 will range in a read mode between 10-25 milliamps and preferably about 20 milliamps, and in a write mode between 30 and 50 milliamps and preferably about 40 milliamps. Those skilled in the art will appreciate that the optimum operating current will vary depending on specific technology used to implement the transistors used.

In one embodiment, the MRAM circuitry 860 may include any useful MRAM memory circuit, as for example, a word line or a bit line. The n-channel transistor 843 may preferably be a CMOS n-channel field effect transistor. The activation signal may preferably comprise a voltage level of at least a logic “1” in order to turn on the n-channel transistor. Voltage levels between logic “1” and logic “0” may be used to control current flow through the n-channel transistor. MRAM circuitry 860 and MRAM 10 is one example embodiment of core circuitry 720.

According to the preferred teachings of the present invention, regulated transistors driving the word lines are system implemented as n-channel and the regulated transistors driving the sense lines are implemented as p-channels. Those skilled in the art will recognize that this configuration has the advantage that noise on the word line drivers is electrically isolated from the sense line transistors.

Refer now to FIG. 8 which shows one example embodiment of the stable reference current generator as embodied in current reference 722 and variable analog control circuitry 714 according to the preferred teachings of the present invention. FIG. 8 shows the global bias setting circuitry 414, the bias circuitry 412, and indicator circuitry 416. Bias circuitry 412 optimizes the bias level set initially by bias setting circuitry 414.

The bias setting circuitry 414 has a stable voltage reference 441 such as a band gap or other suitable circuit that provides a stable voltage or compensated voltage over temperature. The stable voltage source 441 provides a voltage 447 to a stable current source 443 such as a voltage and process compensated current supply. One example embodiment of current reference 722 is the bias setting circuity 414. One example embodiment of variable analog control circuitry 714 is the bias circuitry 412.

The bias setting circuitry 414 that is associated with the bias can be modified by laser trimming of fuses or other devices to optimize bias outputs in a well-known manner. The bias setting circuitry 414 sets the circuit levels for optimizing the MRAM 10. The bias setting circuitry 412 includes circuitry for modifying and fine-tuning the operation of the MRAM 10. This is necessary because during manufacture, the underlying circuitry has a normal variation, from variations in active device operation, such as the MRAM's transistors. For example, this variation may be across a 20% range. The bias circuitry 412 tunes this to a 1%-2% range per die or per wafer because processing or fabricating variations have been tuned out. The global bias setting circuitry 414 provides the global bias signal 415. The global bias signal 415 is one example embodiment of the electrical connections 730.

The bias signal 415 represents the stable operating point of the MRAM 10. The bias signal 415 is provided to the bias circuitry 412. The bias circuitry 412 provides the bias tuning signal 417 to the indicatory circuitry 416.

Transistor 454 is a mirror input for the bias signal 415. This provides a basis for multiplying the bias signal 415 using transistors 456, 458, 460 or 462. Each gate on transistors 456, 458, 460 or 462 is connected by bias tuning signal 417 to the gate of transistor 454 in a mirror configuration. The bias signal 415 represents the stable operating point of the MRAM 10. Transistor 454 is a mirror input for the bias signal 415. Transistor 464 is controlled by signal 417. The internal signals can be ratioed or duplicated through indicator circuitry that isolates these internal signals from external influences. For example, the current gain between transistor 464 and transistor 454 can be set to any desired amount by varying the relative gain between them in a well known manner.

Specific selection of a particular mirror transistor 456, 458, 460 or 462 is done by switches 446, 448, 450 or 452 as decoded by decoder 440. Decoder 440 is controlled by select signal 442 and 444. The switches will connect only one of the outputs of transistors 456, 458, 460 or 462 to the word bias reference current 434. The word bias reference current 434 is one example embodiment of the word reference current 734.

The bias setting circuitry 414 provides a stable signal 415 over temperature and voltage to bias circuitry 412 which generates the word bias reference current 434 that is selected by signal 442 and signal 444. Signals 442 and 444 can be set externally on pad 422, or by chip defaults as is conventionally known. The bias circuitry 412 also outputs the bias tuning signal 417 to the indicator circuitry 416. The word bias reference current 434 is generated from the bias signal 415 and the select signal 442 and 444 and feeds into the standard decoder circuitry 440 where word bias reference current 434 is used to set the magnitude of word current 212, a sense current 216 or any other signal. Each signal desired to be tuned needs a duplicate bias circuitry 412 to drive its bias as well as indicator circuitry 416 and test pad 422. The indicator circuitry 416 is connected to an external pad 422 that, when placed in test mode, will supply the indicator current line 432 that is proportional to the word bias reference current 434. The indicator circuitry 416 is placed in test mode by driving the pad 422 to a high state, or VDD, and indicates the current supplied by the indicator circuitry 416. Each word bias reference current 434 needs its own set of bias circuitry 412.

For example, if the desired test is for 14 times the bias reference signal, the select signal 442 and 444 would be “10” indicating the switch 450, implemented as a transistor, would activate the word bias reference current 434 to allow transistor 460 to drive the word bias reference current 434.

In operation during manufacturing, one or more die could be tested on a wafer and from this testing the correct bias settings can be determined and applied to all die on the wafer. Then, each die on the wafer could be laser trimmed by laser trimming fuses on the die. This process increases wafer yield by optimizing the operation of the delay circuitry or the current sources, either the word current source or sense current source.

Circuitry is provided to change the levels using a register that can be changed. The test/optimization cycle involves changing the settings and testing the MRAM. A shift register method is used to set where the bias is. The bias can be set using conventional methods or as shown with reference to FIGS. 7, 8 and 9.

Refer now to FIG. 9 which shows a method to test and set operating points on an MRAM 10. As diagrammatically indicated in 480, the test pad 422 is connected to either VDD or ground depending on how the indicator circuitry is configured. For example, on N-channel transistor 416, the test pad 422 is connected to VDD, and for a P-channel transistor 416, the test pad 422 is connected to VSS or ground. Next, the current in indicator current line 432 is measured, as diagrammatically indicated in 482, by using well known external current measuring devices such as automated current meter in test equipment. The MRAM 10 is tested, as diagrammatically indicated in 484, to determine if the correct logical operation of the device by writing and reading back data to the MRAM 10. The current in indicator current line 432 and the memory operation is noted, as diagrammatically indicated in 486. Then, the magnitude of the word current 212 is tuned, as diagrammatically indicated in 490, by adjusting the word bias reference current 434 using the bias circuitry 412 by selecting the desired setting or range of settings using the select signals 442 and 444 and producing the bias tuning signal 417. Then the test sequence is repeated as often as desired until all desired operating points are set and recorded.

The best setting for the MRAM 10 on a particular wafer is noted by determining the most consistent and accurate results by writing and reading the MRAM 10. Then, the other operating points of the MRAM 10 can utilize the best operating point discovered by the procedure according to the preferred teachings of the present invention, for the portion of the MRAM 10 or wafer or batch that this MRAM 10 represents.

Thus, the operating point of the MRAM 10 is adjusted for variations in production and manufacturing.

The invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles of the present invention, and to construct and use such exemplary and specialized components as are required. However, it is to be understood that the invention may be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment details and operating procedures, may be accomplished without departing from the true spirit and scope of the present invention. 

1. An apparatus for improving stability of a magnetoresistive random access memory over process and operational variations comprising, in combination: a current reference circuit for providing a reference current control signal; variable analog control circuitry connected to receive the reference control signal from the current reference circuit, where the variable analog control circuitry generates a word current reference signal in response to the reference current control signal and further generates a source current reference signal in response to the reference current control signal; at least one word current source connected to receive the word current reference signal; and at least one sense current source connected to receive the source current reference signal.
 2. The apparatus of claim 1 further comprising in combination: a bias tuning circuit comprising, in combination: a bias generator having a bias output; a plurality of switches having a word reference input and a mirror transistor output; a plurality of mirror transistors connected to one of the mirror transistor outputs; a transistor connected in a mirror configuration with the plurality of mirror transistors having a tuned reference output; and a selector to select one of the mirror transistor to activate the transistor to set the voltage to the plurality of mirror transistors.
 3. The apparatus of claim 2 with each one of the plurality of mirror transistors having a different gain.
 4. The apparatus of claim 2 further comprising, in combination: a pad; and a indicator transistor in a mirror configuration with the transistor connected to the pad to provide an indicator.
 5. The apparatus of claim 4 with the pad being an external pad.
 6. The apparatus of claim 4 with the indicator transistor having a gain that is a multiple of the transistor.
 7. The apparatus of claim 2 with the plurality of mirror transistors are n-channel transistors.
 8. The apparatus of claim 2 with the plurality of switches being transistors.
 9. The apparatus of claim 2 with the transistor being an N-channel transistor.
 10. The apparatus of claim 2 with the bias generator being a temperature and voltage compensated bias generator.
 11. The bias turning circuit of claim 2 with the selector selecting one of the plurality of mirror transistors to compenstate of a tested parameter.
 12. The apparatus of claim 2 with the tested parameter being manufacturing variance.
 13. The apparatus of claim 1 further comprising in combination: a word current source comprising, in combination: a control circuit having a control input; an n-channel transistor including a gate, a source and a drain, where the source is coupled to a supply ground, the drain is coupled to the magnetoresistive random access memory circuit and the gate is connected to the control input; and a positive supply voltage, coupled to the magnetoresistive random access memory circuit so as to allow current to flow through the magnetoresistive random access memory circuit when an activation signal is applied to the gate by the control circuit.
 14. The apparatus of claim 1 wherein the control circuit comprises a voltage regulator that regulates a voltage level at the gate, so as to limit an amount of current flowing through the n-channel transistor and the magnetoresistive random access memory circuit.
 15. The apparatus of claim 14 with the voltage regulator comprising a feed back amplifier.
 16. The apparatus of claim 14 with the voltage regulator being current controlled.
 17. The apparatus of claim 13 with the n-channel transistor comprising a complementary metal oxide semiconductor n-channel field effect transistor.
 18. The apparatus of claim 17 wherein the control circuit comprises a current regulator that regulates current flowing through the complementary metal oxide semiconductor (CMOS) n-channel field effect transistor when the CMOS n-channel field effect transistor is turned on and off.
 19. The apparatus of claim 13 with the control circuit comprising a stabilization amplifier.
 20. The apparatus of claim 19 with the stabilization amplifier further comprising, in combination: a logic control having a read/write input, an on/off input, a write reference gate control signal and a read reference gate control signal; a read reference switch having a read reference input and a reference output, with the read reference switch having a read reference control connected to the read reference gate control signal; a write reference switch having a write reference input and a write reference output connected to the reference output, with the write reference switch having a write reference control connected to the write reference gate control signal; and a feedback amplifier connected to the reference output, having a mirror current output and a mirror feedback voltage input.
 21. The apparatus of claim 13 with the control input being a regulated mirror gate signal.
 22. The apparatus of claim 1 with the sense source further comprising, in combination: a reference transistor having a gate, a current reference input and a voltage supply input; and a plurality of transistors in a mirror configuration with the reference transistor with each gate of the plurality of transistors connected to the gate, with the plurality of transistors each providing a sense current.
 23. The current controlled current source of claim 22 with the plurality of transistors being P-channel transistors.
 24. The apparatus for improving stability of a magnetoresistive random access memory of claim 1 wherein the current reference circuit comprises a bandgap voltage reference that feeds into and controls a stable current reference. 